Mercury in Isopods and Steelhead Trout

My name is Roberto Rivera and I am an undergraduate at the University of California Santa Cruz with a focus in chemistry. I have been awarded the 2020 Keeley Coastal Scholars Award, which will be used to fund my project in determining if non-native terrestrial isopods are transferring mercury to the steelhead trout population via consumption in Big Sur.

The southern range of steelhead trout (O. mykiss) in the Big Sur region is a threatened species due to a small population and one that is vulnerable to the impacts of climate change, non-native species, and accumulation of toxins. Researchers from The National oceanic and Atmospheric Administration (NOAA) have found that some resident fish in Big Creek, a small watershed just south of the town of Big Sur, had strontium to calcium ratios in their ear bones showing results as if they swum to the ocean, yet the barium to calcium ratios confirmed they had not. These researchers had also found that a subset of these fish consume a large proportion on non-native terrestrial isopods (pill bugs) as part of their diet, which led them to hypothesize that the excess strontium could be from marine fog, which is known to carry aerosols derived from seawater. This would mean that the pill bugs are picking up the strontium of marine origin and transferring it to the fish.

Cygnus921, Flicker, December 9,2009

NOAA Fisheries


Methylmercury (MeHg) is also present in marine fog in California, and as a powerful neurotoxin, MeHg could be negatively impacting the fish if it is getting into the pill bugs along with the strontium. Previous research has demonstrated that pill bugs contained almost 60% of their mercury in the methylated form (MeHg), a higher proportion than other insects. Thus, our hypothesis is that the non-native terrestrial isopods (pill bugs) transfer marine-derived Hg to threatened steelhead trout in the Big Creek watershed.

By collecting terrestrial isopods, steelhead trout, and sculpin fish as controls (they do not consume terrestrial isopods) in Big Creek, one of the University of California Natural Reserves, under the supervision of scientist with NOAA National Marine Fisheries service, the samples can be tested for total Hg and MeHg concentrations. Data will then be analyzed to look for anomalously high concentrations of total Hg and MeHg in the isopods and the steelhead, and compare these to the control samples, the sculpins. The results will then determine whether the steelhead trout might be experiencing sub-lethal effects due to Hg contamination and what the sources of this Hg might be.